In the installation of a tubular body, such as a large pipe, in such facilities as nuclear power plants and large-scale plants, a problem is tensile stress remaining in the pipe after welding. Welding causes residual tensile stress in the inner surface of a pipe, and the residual tensile stress may possibly shorten the life of the pipe. For this reason, it is desirable that such residual stress caused by welding be improved so as to be turned to compressive stress.
An induction heating stress improvement process (hereinafter, referred to as an IHSI process) is proposed as a method for improving residual tensile stress in a pipe. The IHSI process is carried out as follows. Firstly, while the inner surface of a pipe is forcibly cooled by running water, the temperature of the pipe is raised by being inductively heated from the outer-surface side using a high-frequency induction heating coil. The pipe thus heated has a temperature gradient in the wall-thickness direction near a part satisfying stress corrosion cracking (hereinafter, referred to as SCC) conditions of the pipe. Thereafter, the heating of the pipe is stopped, but the cooling continues by running the water along the inner surface until the pipe has a substantially uniform temperature in the wall-thickness direction of the pipe. As a result, residual stress that has been tensile around the welded part is reduced or is turned to be compressive (see Patent Documents 1 to 3).
The following are some of the other methods proposed for improving residual tensile stress in a pipe. In a method, a surface of a pipe made of, for example, a stainless steel is heated to the solution heat-treatment temperature, or alternatively is melted by laser irradiation so that the residual stress in the backside surface of the pipe can be reduced. In another method of reducing residual stress, a pipe is heated by linearly irradiating the outer surface of the pipe with laser beams while the laser is rotationally moved (see Patent Documents 4 to 8).
Patent Document 1: JP-A-57-70095
Patent Document 2: JP-A-2001-150178
Patent Document 3: JP-A-10-272586
Patent Document 4: JP-A-2003-004890
Patent Document 5: JP-A-8-5773
Patent Document 6: JP-A-2000-254776
Patent Document 7: JP-A-2004-130314
Patent Document 8: JP-A-2005-232586